Ca influences nitrogen assimilation and protein metabolism
Plant growth requires a source of nitrogen for the biosynthesis of a
variety of nitrogen-containing biomolecules, such as amino acids and
nucleic acids. Nitrogen deficiency leads to crops productivity and
quality reduction.43Previous study has reported Ca mediated nitrogen assimilation in
plant.44 Our study inP. massoniana revealed that exogenous Ca may activate the
expression of glutamine synthetase (GS; spots 9 and 10), and motivate GS
mediated nitrogen assimilation
pathway.45
Apart from nitrogen assimilation related protein, a number of protein
metabolism related proteins were detected with changed expression
patterns at different Ca levels (Table 1), which involved in protein
synthesis, folding, destination and degradation. The up-regulated mRNA
translation and peptide elongation related proteins asparagine-tRNA
ligase (spot 49) and glycine-tRNA ligase 1 (spot 50) under high Ca
condition suggests Ca perhaps benefit the binding of amino acid and tRNA
and contribute to protein biosynthesis. The increased soluble protein
content at high Ca level (Figure 1F) is an evidence for this
speculation.
Four heat shock protein (HSP) 70 related proteins were identified
(spot54-56, 64), whose abundances showed markedly increment at high Ca
level (Table 1). HSP 70 is a diverse function protein. Apart from stress
response, it also involves in plenty of other biological processes, such
as protein folding, assorting and translocation, and the development and
differentiation of various plant
tissues.46Consistently, HSP70 owns the highest degree in the PPI network (Figure
5), showing its versatile roles in diverse biological processes.
Adequate Ca treatment stimulated HSP 70 high expression possibly
facilitates the aforementioned biological processes in P.
massoniana seedlings, especially protein folding and transportation
related process.
Peptidyl-prolyl cis-trans isomerase (PPIase) catalyzes the reversible
conversion of the peptidyl-prolyl bond from cis to trans, which is a
rate-limiting step in the folding of
proteins.47 Previous
study in Arabidopsis reported overexpression of PPIase gene
confers stress tolerance to heat, ABA, drought and
salt.48 Our study inP. massoniana showed the expression of PPIase CYP20-2 increased
ten folds under Ca deficiency condition (Table 1). Up-regulated PPIase
may be a potential approach for P. massoniana to cope with Ca
deficiency, whose underlying mechanism is associated to PPIase in
correct protein folding. It is noticeable that high Ca treatment induced
drastically soaring of PPIase CYP20-2 as well. This phenomenon can be
attributed to high Ca improved protein synthesis and seedling growth,
more PPIase is necessary to ensure the proper folding and packaging of
accelerating synthesized proteins.